Published literature from manufacturers and users teaches that the bearings of AC induction motors powered from variable frequency drives are adversely affected by electrical current which circulates through the motor shaft and the bearings. In recent years, the incidence of premature bearing failures in AC induction motors powered from variable frequency drives has been increasing steadily, especially since the introduction of faster switching power electronics which allows better speed control by operating at higher operating frequencies utilized to generate sinusoidal waveforms. Because variable frequency drives use pulse switching techniques to provide a sinusoidal waveforms of variable frequency which is used to feed the motor stator field coils, the presence of faster switching waveforms allows more currents to be generated in the motor rotor, such current being available to circulate to ground by going through the bearings. The mechanism of failure of the bearings is identified as electrical arcing between the bearing races and its rotating balls or rollers. When electrical arcing occurs between the inner or outer race of a bearing, the energy in the electrical arc creates tiny pits in the bearing race, thereby initiating a self-sustaining mechanical destructive sequence where the pits generate more possibilities of arcing because of the surface deterioration of the metal.
In response to a continuously increasing number of electrical current related bearing failures in motors, the industry has developed a number of bearing current mitigating techniques associated with the utilization of variable frequency drive driven motors.
Stator coil design solutions involve reducing bearing current levels through coil design. The level of current made available to flow through the bearings of an AC motor is affected by the balancing of the magnetic field generated by each of the stator coils. Coil design solutions which are aimed at reducing the level of available bearing current have practical limitations. For AC induction motors, the limitations imposed on the design of field coils and their magnetic cores which generate very low levels of bearing current is the physical and electrical configuration of the field coils. Coil and core design options in motors are restricted by the need to provide electrical windings wound in physically opposite positions around the periphery of the motor frame. Winding and core design which would insure that no shaft current is generated in the rotor has been so far impossible to realize. Bearing electrical isolation solutions is another bearing current mitigating approach which has been developed. This involves coating the outer housing of the bearing, most often using plasma coatings to deposit a thin layer of ceramic type material displaying a high ohmic resistance. Unfortunately, the insulating coatings materials are brittle and thus are subject to loss of isolation due to the brittle ceramic coating added to bearing housings. The same may be said of bearings using ceramic coated steel segments.
Strategic equipment grounding techniques is yet a further solution to reduce the negative effects of bearing current. The goal of strategic grounding is to provide grounding paths which tend to minimize the level of available bearing current. The effectiveness of strategic equipment grounding techniques is subject to vary in time as electrical equipment is modified or added to new machinery and equipment in the electrical circuits attached to variable frequency drive of the motor. By providing new or different paths for the magnetic field to generate bearing currents, the current mitigating efficiency of strategic grounding locations is eventually nullified.
Shaft grounding techniques are yet another possibility for reducing the effect of bearing current is to provide a path for the current to flow to ground before reaching the bearing. This requires the installation of grounding brushes installed on the motor shaft. The use of grounding brushes has limitations regarding the level of shaft current it can carry to ground while preventing shaft voltage to rise significantly. The positioning of the grounding brushes is also critical in preventing a parallel current path through the bearing. Finally, the performance of the brushes diminishes as they wear and as dirt and other contaminants negatively affect the electrical resistance of the grounding brushes.